Gas shales are economically viable hydrocarbon prospects that have proven to be successful in North America. Unlike conventional hydrocarbon prospects, gas shales serve as the source, seal, and the reservoir rock. Generating commercial production from these unique lithofacies requires stimulation through extensive hydraulic fracturing. The absence of an accurate petrophysical model for these unconventional plays makes the prediction of economic productivity and fracturing success risky.This paper presents an integrated approach to petrophysical evaluation of shale gas reservoirs, specifically, the Barnett Shale from the Fort Worth basin is used as an example. The approach makes use of different formation evaluation data, including density, neutron, acoustic, nuclear magnetic resonance, and geochemical logging data. This combination of logging measurements is used to provide lithology, stratigraphy and mineralogy. It also differentiates source rock intervals, classifies depositional facies by their petrophysical and geomechanical properties, and quantifies total organic carbon. The analysis is also employed to locate optimal completion intervals, zones preferable for horizontal sections, and intervals of possible fracture propagation attenuation. Resistivity image analysis complements the approach with the identification of natural and drilling induced fractures. We compare results from three different wells to show the effectiveness of the method for shale gas characterization.The methodology presented provides a means to understand the geomechanical and petrophysical properties of the Barnett Shale. This knowledge can be used to design a selective completion strategy that has the potential to reduce fracturing expenses and optimize well productivity. Though developed specifically for the Barnett Shale, the underlying ideas are applicable to other thermogenic shale gas plays in North America.
[1] An accurate description of water-or oil-bearing reservoirs strongly depends on a robust determination of their petrophysical parameters, e.g., porosity, permeability and fluid distribution. Downhole logging measurements are the primary means to formation evaluation; however, they do not directly provide the petrophysical properties of interest. To interpret well logging data, a range of empirical models are usually employed. These empirical relationships, however, lack scientific basis and usually represent generalizations of the observed trends. Since macroscopic rock properties vary depending on their microstructure, we suggest using a pore-scale approach to establish links between various petrophysical properties of sedimentary rocks. We outline a method for computing formation permeability using the proposed rock models. The method utilizes NMR (Nuclear Magnetic Resonance) logging data for the information about porosity and grain size. We also present an approach for prediction of acoustic velocities of model rocks. The proposed methodology is applied to the field data, and the corresponding interpretation results are included in this paper.Citation: Gladkikh, M., D. Jacobi, and F. Mendez (2007), Pore geometric modeling for petrophysical interpretation of downhole formation evaluation data, Water Resour. Res., 43, W12S08,
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AbstractUsing an interfacial tracer technique, our experiments show qualitatively different trends of total interfacial area between the wetting and non-wetting phases as a function of saturation, depending on whether the system is strongly or weakly wetted. A strongly wetted system is defined as one in which the wetting phase can spread as a thin film on the solid surface. We assess the relative contributions of fluid/fluid and fluid/solid interfaces to the total area using thermodynamic arguments. The fluid/solid contribution to area plays a crucial role in explaining the measurements. The influence of interfacial area on relative permeability is not straightforward. Simple analysis based upon pore-level distribution of phases in a model porous medium allows quantifying the differences in the relative permeabilities for both weakly and strongly wetted systems, measured simultaneously with the interfacial area. Relative permeability correlates with fluid/solid area but not with fluid/fluid interfacial area.
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